1. Field of the Invention
The present invention relates generally to deflection convergence systems for multi-gun shadow mask cathode-ray tubes (CRTs), and more particularly to an improved lateral convergence system for such tubes.
2. Description of the Prior Art
The color CRTs used in most present-day television and similar color image display systems have three electron guns--one for each of three primary colors (red, green and blue). The guns are disposed symmetrically in a triangular or "delta" arrangement around the central axis of the tube, and generate individual electron beams that pass through small holes in a shadow mask positioned between the guns and a cathodoluminescent display screen. The display screen is formed of three color phosphors deposited in a regular pattern of dots on the inner face of the CRT. The relative locations of the phosphor deposits and shadow mask apertures is such that, ideally, each electron beam strikes only the deposits of its respective color. An electromagnetic deflection yoke positioned between the electron guns and the screen deflects the three beams over the screen surface. To produce color images correctly, the three electron beams must be coincident at the screen's surface in the absence of a deflecting field (static convergence) and under all conditions of deflection (dynamic convergence).
Static convergence may be achieved by applying fixed magnetic fields to one or more of the beams using suitable external magnets. To maintain the three beams in coincidence during scanning of the screen, suitable waveforms generated in synchronization with the horizontal and vertical deflection signals are applied to individual red, green and blue convergence coils mounted on the CRT neck. The currents produced in these coils by suitable drivers move each beam radially to achieve dynamic convergence.
To obtain complete convergence of the three beams, however, an additional degree of freedom is required. For this purpose a fourth convergence waveform may be applied to a dynamic lateral convergence coil assembly to shift one beam (usually the blue beam) horizontally relative to the others. Such "dynamic blue lateral" convergence is particularly needed with high resolution shadow-mask CRTs, because of the superior convergence accuracy they require.
Prior art lateral convergence correction systems move the blue beam horizontally in one direction while moving the red and green beams in the opposite direction. Such a system is described, for example, in J. S. Beetson et al., IBM J. Res. Develop., vol. 24, no. 5, September 1980, pp. 598-611. While such an approach seems, in theory, to be satisfactory, it is difficult to implement because of the requirements that the red and green beams not separate while they are being shifted laterally, and that the blue beam not move vertically with respect to the red and green beams. They also require a complex magnetic field structure, which is costly to design and produce.